1. Field of Invention
The present invention generally relates to a method and apparatus for objectively assessing in vivo the properties of ocular tissue, and more particularly to a method and apparatus for in vivo imaging and analysis of corneal tissue in an objective and quantitative manner for diagnostic and therapeutic purposes.
2. The State of the Art
In many diagnostic and therapeutic applications, there is great need to objectively quantify the optical density, shape and size of various ocular tissue, such as the crystalline lens and cornea.
Regarding the cornea lens, it is well known that the presence of corneal haze at particular locations on the cornea can effect, in particular individuals, the visual acuity and function of the eye. It is also known that the optical density of the corneal haze is related to the amount of light diffusion (ie., scatter) caused by increased size and coagulation of protein molecules in the cornea. Presently, prior art descriptions of corneal haze have generally consisted of a morphologic statement. Such morphologic descriptions have been based primarily on the patient's potential visual acuity estimated using an acuity scope.
A number of techniques have been developed over recent years for achieving desired refraction-corrective surgery in the human eye by laser sculpturing the optically used portion of the cornea. Examples of such prior art techniques are disclosed in U.S. Pat. Nos. 4,718,418; 4,732,148; 4,773,414; 4,729,372; 4,669,466; 4,473,330; 4,856,513; and 4,994,058. While some of these U.S. Letters patents utilize different forms of apparatus, they each disclose essentially the same method for photoablative laser sculpture of the cornea.
In general, the preoperative step of the method involves removing the epithelial layer from central anterior area of the cornea. Then an ultraviolet laser beam of a controlled cross-sectional diameter is directed to the epithelium free area for uniform photoablation through the Bowman's membrane and selective penetration of the stroma, achieving a new curative profile of predetermined characteristics solely in stroma tissue. Thereafter, post operative procedures favorable to smooth efficient epithelial regrowth over the surgically sculptured region are performed. As it is not presently uncommon for a certain amount of corneal "haze" or light scattering to result from the laser sculpturing procedure, which may be more or less noticeable in different patients, post-operative treatment of this disorder is also typically performed using a variety of typically applied drugs.
Prior to performing the corneal sculpturing procedure described above, it is important to acquire data representative of thickness and topography of the cornea of a particular abnormal eye. Such data must be in the form of a readily interpretable context against which the depth and surface distribution of the surgical incision into the anterior surface of the abnormal cornea can be determined in order to achieve a desired refractive correction in the patient's eye. In addition, for medical and legal documentation purposes, it is important for the ophthalmological surgeon to objectively determine and record the precise degree of corneal haze present in the patient's eye prior to and after laser sculpture of the cornea.
U.S. Pat. No. 4,669,466 discloses a CAD/CAM system for use in acquiring corneal topographical and thickness data which can be used by the ophthalmological surgeon in determining the new curvature profile to be formed in the stroma in order to achieve a desired degree of optical correction in the patient's eye. Equipment presently used for acquiring corneal topographical data includes an optical ocular scanner or a photokeratometer with provision for generating digitized topographical data. Exemplary of this equipment is the PFS-1000 photokeratoscope commercially available from the Japanese firm, Sun Contact Lens Co., Ltd., with U.S. offices in Palo Alto, Calif. The Sun photokeratoscope has the ability to rapidly scan the cornea in such a way as to determine the entire topography of the outer surface of the cornea, from limbus to limbus. Subtle differences in curvature of the outer cornea or inner optical zone are precisely and clearly defined. The photokeratoscope is available with a photoanalyzer having the capability of digitizing the data from thousands of individual points on the particular cornea, and producing a digitized output, from which a visual display is producible to show the cross-sectional profile of anterior-surface curvature for any cross-sections which include the central axis of the eye.
Equipment presently used for acquiring corneal thickness data includes pachymeter for making multiple determinations of the precise thickness of the cornea, to within thousandths of a millimeter, at plural locations on the surface of the cornea. Using ultrasonic-ranging, measured thickness data correlated with location-coordinate data is provided as digitized output. The pachymeter measurements may be performed manually on an individual point-by-point basis, using a commercially available hand-held transducer probe flexibly connected to power supply and display means, for example the Myopach ultrasonic pachymeter available from Myocure, Inc., Los Angeles, Calif., or the "Villasenor" ultrasonic pachymeter, available from Cilco, Inc. Huntington, W. Va. In using such a device, a fixation target enables the unexamined eye of the patient to maintain central-axis stability for his examined eye when the probe is placed on the corneal surface anywhere from the central optical axis to the periphery.
Various patents that describe apparatus for measuring ocular tissue are described in the following U.S. Pat. Nos.: 5,684,562; 5,634,249; 5,673,109; 5,624,436; 5,620,437; 5,470,329; 5,596,377; 5,562,656; 5,507,799; 5,549,599; 5,488,443; 5,139,022; 5,404,884; 5,800,424; 5,784,146; 5,817,088; 5,726,735; and 5,706,072; the disclosures of which are incorporated herein by reference.
While the above-described equipment has the capability of acquiring topographical and thickness data of the cornea, the nature of this data is approximate as it has been generated on the basis of a fixed number of measurements made at points along the surface of the cornea and then applying mathematical estimation techniques.
Thus, there is great need for a method and apparatus that is capable of producing objective measurements of corneal haze and determining the 3-D geometry of the cornea and associated structures in a way which is free from the shortcomings and drawbacks accompanying the prior art.
Accordingly, it is primary object of the present invention to provide a method and apparatus for in vivo imaging and analysis of corneal tissue in an objective, quantitative manner.
It is a further object of the present invention to provide such a method and apparatus, from which cross-sectional images of corneal tissue can be formed over a high depth of field extending far beyond the thickness of the cornea and crystalline lens.
A further object of the present invention is to provide such a method and apparatus, from which accurate cross-sectional images of corneal tissue can be formed, with correct spatial relationships between ocular structures.
A further object of the present invention is to provide a method and apparatus for precisely measuring the physical dimensions of the cornea and its correct spatial relationships within the eye.
An even further object of the present invention is to provide a method and apparatus for forming cross-sectional images of corneal tissue which enable precise localization of zones of increased optical density, such as corneal haze.
Yet a further object of the present invention is to provide a laser-based corneal tissue analysis system in which cross-sectional digital images of the cornea, crystalline lens and surrounding ocular structures can be formed and from which the precise degree and location of optical density of the cornea can be objectively determined using digital image analysis.
A further object of the present invention is to provide such a corneal tissue analysis system in which the luminance and cross-sectional dimension of the laser illumination used to visualize the lens and form cross-sectional corneal images, can be maintained essentially uniformly constant from image to image, and photo-examination session to photo-examination session.
A further object of the present invention is to provide such a corneal tissue analyzing system which includes a microscope and an image detector that uses laser illumination for visualizing and forming perfectly focused cross-sectional images entirely through the outer tissue comprising the cornea and crystalline lens.
An even further object of the present invention is to provide a laser-based corneal tissue analysis system in which 3-D model of the cornea and its surrounding ocular structures in the eye can be generated using cross-sectional digital images formed of these structures.
These and other objects of invention will become apparent hereinafter and in the claims.